Late Palaeozoic Tetrapod Biogeography
Final Report Abstract
To analyse the biogeography of Palaeozoic tetrapods using modern quantitative approaches, the project was divided into multiple parts. The first included anatomical descriptions and phylogenetic analyses of selected key taxa, such as the possible caseid Phreatophasma aenigmaticum and the alleged varanopids Milosaurus mccordi and Pyozia mesenensis. The reassessment confirmed Phreatophasma as basal caseid, not closely related to the other Russian caseid Ennatosaurus tecton, suggesting multiple dispersals from equatorial to temperate latitudes in caseids. Restudy of Pyozia supported its assignment to Varanopidae, indicating yet another dispersal event from equatorial to palaeotemperate latitudes. In contrast, Milosaurus was identified as basal member of Sphenacomorpha, not providing novel biogeographic signals, but interestingly with its large size documenting another independent evolution of large body size in synapsids. In addition, the event-based likelihood method BioGeoBEARS has been applied to a supertree of 593 Carboniferous and Permian tetrapods divided into 1 to 13 bioregions to quantitatively deduce the occurrence, timing, and effect of the four classes of events: 1. vicariance, 2. dispersal, 3. sorting, and 4. sympatry. The dispersal and vicariance rates inferred using this new method indicate two major shifts in dispersal rate: a decrease towards the end of the Carboniferous, and a further decrease at the end of the middle Permian. Both decreases in dispersal rates were accompanied by peaks in vicariance. The Carboniferous decrease was primarily driven by amphibians (amniotes show little change), and their dispersal rates remained low throughout the Permian. Conversely, the middle Permian decrease in dispersal rate was observed primarily in amniotes. In an occurrence analysis, beta diversity of Palaeozoic tetrapods was investigated by proposing and implementing a novel approach towards defining bioregions, resulting from the realization that the spatial scale in the study of biogeography is of particular importance and has previously been neglected in palaeontological studies. A newly created Palaeozoic tetrapod dataset was subjected to two hierarchical cluster analyses. The first groups the formations based on geographical distances between them, the second based on taxonomic distances. Clusters shared between the two sets represent geographically continuous areas of endemicity and were used as bioregions in the analysis and the spatial scale, at which the bioregions were defined, was standardized over geological time. The results emphasized the increased importance of continental-scale barriers, with the increased orogenic uplift as a possible control, rather than local scale, and the previously suggested changes in vegetation caused by the rainforest collapse could be rejected as a driver of increased provinciality. Moreover, we used an up-to-date biostratigraphy and incorporated recent discoveries to thoroughly review the Permian tetrapod fossil record. Taxonomic distance metrics were used to compare faunas in the equatorial latitudes to their contemporaries in temperate latitudes, to see if these distances were higher or lower than the faunal turnover through time. Sampling correction was also applied to test whether the modern latitudinal diversity gradient was observed in the Permian. It was found that the faunal transition (from pelycosaur-dominated to therapsid-dominated faunas) represents a genuine mass extinction and not an artefact of a latitudinal biodiversity gradient or shift in sampling localities. Sampling correction demonstrates an inverse latitudinal biodiversity gradient in the Permian, with peak diversity in temperate latitudes. Finally, a database of the well-sampled South African Karoo Basin was used as a case study to investigate provinciality within a single basin. The results show that beta diversity correlates significantly with the evenness of the abundance distribution, even when subsampling to correct for incomplete sampling is applied. This is a particular problem when examining the end-Permian mass extinction in the Karoo, where the disaster taxon Lystrosaurus shows extreme dominance and produces an extremely uneven abundance distribution. Therefore, a new method, dubbed the Relative Abundance Corrected (RAC) beta diversity, was designed to correct for the influence of changes in the evenness of the abundance distribution. The results show that, contrary to previous studies, there was no increase in cosmopolitanism across the end of the Permian. Instead, beta diversity increased, indicating a more provincial fauna following the mass extinction.
Publications
- 2017. A re-examination of Milosaurus mccordii, and the evolution of large body size in Carboniferous synapsids. Society of Vertebrate Palaeontology Annual meeting: Calgary, Canada
Brocklehurst, N. & Fröbisch, J.
- 2017. A re-examination of the enigmatic Russian tetrapod Phreatophasma aenigmaticum, and its evolutionary implications. – Fossil Record (80): 87-93
Brocklehurst, N. & Fröbisch, J.
(See online at https://doi.org/10.5194/fr-20-87-2017) - 2017. Olson's Extinction and the latitudinal biodiversity gradient of tetrapods in the Permian. – Proceedings of the Royal Society B (284): 20170231
Brocklehurst, N., Day, M.O., Rubidge, B.S. & Fröbisch, J.
(See online at https://doi.org/10.1098/rspb.2017.0231) - 2018. A re-examination of Milosaurus mccordii, and the evolution of large body size in Carboniferous synapsids. – Journal of Vertebrate Paleontology
Brocklehurst, N. & Fröbisch, J.
(See online at https://doi.org/10.1080/02724634.2018.1508026) - 2018. Accounting for differences in species frequency distributions when calculating beta diversity in the fossil record. – Methods in Ecology and Evolution (9): 1409-1420
Brocklehurst, N., Day, M.O. & Fröbisch, J.
(See online at https://doi.org/10.1111/2041-210X.13007) - 2018. Physical and environmental drivers of Paleozoic tetrapod dispersal across Pangaea. – Nature Communications (9): 5216
Brocklehurst, N., Dunne, E.M., Cashmore, D.D., & Fröbisch, J.
(See online at https://doi.org/10.1038/s41467-018-07623-x) - 2018. The definition of bioregions in palaeontological studies of diversity and biogeography affects interpretations: Palaeozoic tetrapods as a case study. Society of Vertebrate Palaeontology Annual meeting: Albuquerque, USA
Brocklehurst, N. & Fröbisch, J.
- 2018. The definition of bioregions in palaeontological studies of diversity and biogeography affects interpretations: Palaeozoic tetrapods as a case study. – Frontiers in Earth Sciences (6): 200
Brocklehurst, N., & Fröbisch, J.
(See online at https://doi.org/10.3389/feart.2018.00200)
